Simon Callow has pursued a career as a writer in tandem with his theatrical profession since his student days in 1969. He has brought to both generous helpings of enthusiasm, ebullience, a gargantuan capacity for work, courage, and a steady proselytising on behalf of actors. His precocious memoir, Being an Actor, written after only nine or so years' practice, was produced after the relative failure of a West End show, The Beastly Beatitudes of Balthazar B, in which he had made a huge investment of good faith and optimism. He says, "I felt liberated . . . from my driven need to create another part, strut my stuff again, earn approval. There was, I knew, a life elsewhere . . . "

He has returned to the theatre periodically as actor, producer and director of both plays and opera, for which he was made a CBE in 1999, but the life elsewhere gave rise, as he says, to hundreds of thousands of written words including biographies of Charles Laughton, Orson Welles and Oscar Wilde, a tender memoir of his friendship with the literary agent Peggy Ramsay and a hilarious account of his work with the east European director Makavejev, Shooting the Actor, which had me laughing out loud.

My Life in Pieces reproduces some of his occasional journalism – reviews, previews, programme notes and obituaries, strung along a narrative of his life that cleverly links the pieces without going over too much old ground. He is never less than generous as reviewer, fellow actor or collaborator, having a capacity for intimacy that goes beyond the subjective. For instance, his thumbnail sketches of the directors Mike Nichols and Michael Rudman are full of love and delight – Nichols "operated by wit and irresistible intelligence and was unusually available to all comers . . . Why do so many directors scream, he had asked himself? Because . . . they feel they're at the very top of the pile, and yet things still don't happen the way they want them to. So, says Mike, he no longer screams. He prepares, to the last degree, but once he comes to shoot the scene, he arrives with a perfectly empty mind, and is generally delighted by what he sees."

And on Rudman:

"I realised more and more that under that raffish and languid exterior beat the heart of a true theatre romantic. This . . . is rare – too rare. Among directors there are visionaries . . . there are careerists . . . But there are few who deeply and tenderly love the theatre and love actors."

Callow shines with love for the business, and is good at the deft phrase that places someone affectionately, but critically – thus Verity Bargate, who ran the Soho Poly theatre in the early days of the London fringe, had "the courage to be indiscriminating".

There is an acute comparison of the qualities of Gielgud and Richardson in a review of new biographies of each, while Laurence Olivier, who was a huge influence both as role model and provider of his first paid employment in the Old Vic box office, (and who has more entries than anyone else in this collection), is nevertheless nailed as "annihilating the competition" and leaving a vacuum when he went.

In the 90s, Callow was, he tells us, the Evening Standard's unofficial gay reporter, having bravely come out in his first memoir, and among the strengths of the present volume are sympathetic portraits of gay icons such as John Gielgud, Frankie Howerd and Nigel Hawthorne. He is often writing from personal knowledge, and some get a drubbing – Rupert Everett is one, and so is the director John Dexter, who gave Callow his early break when he cast him as Mozart in Peter Shaffer's Amadeus at the National Theatre. I love to think of Shaffer, who was sick of being referred to as "Ruby" by Dexter, one day saying: "Now, look here, Rose . . . "

"Rose?"

"Yes: as in 'Rose, thou art sick'."

Perhaps the most original and insightful pieces collected in this volume are the portraits of those among the theatrical great who were sexually ambiguous. Michael Redgrave, Alec Guinness and Denholm Elliott (who once called what he did "dressing up for Mummy and Daddy") emerge from these portraits as complex and driven artists, fuelled in part by that sexual tension at the heart of their identity. It is almost impossible to nail an actor by discussion, because, Callow reminds us, of "the sheer bloody difficulty of acting", but I came away feeling that I understood many of those who populate these pages better even than my own acquaintance with them afforded. That is a great gift.

His view from both sides of the fence, as actor and director, yields many fruits, and there is a pleasing wit to many of these accounts. I loved his piece on Miloš Forman encouraging him to be "strong, true, netcheral" while controlling his every action, and in the same piece the self-deprecating humour of Callow's description of his own "Usual winter costume: a sweep-ing black fedora, an ankle-length black overcoat, and a bright red carnation in my button-hole." At the same time, there is a thread of melancholy running through this volume, rarely articulated, yet informing the tone and moderating the theatricality of his performing self.

A person of gargantuan capacity, as actor, director, biographer, scholar, solo performer, raconteur and confessional writer, never afraid to take on large-scale projects, from a performance of all the sonnets, to encapsulating Dickens alone on stage, to reading absolutely everything written by or about anyone who takes his interest, perhaps the only role to which Callow has not yet applied his multiple skills is as a dramaturg, like another early hero Kenneth Tynan. I hope one day soon his dream of actor-led companies will come true, with him at the helm of one as dramaturg, and perhaps giving us in due course his Falstaff and his Lear, then telling us later all about the experience.

Diana Quick's A Tug on the Thread: From the British Raj to the British Stage is published by Virago.